How do I compute the structural similarity between sentences?
Firstly, before we commence I recommend that you refer to similar questions on the network such as https://datascience.stackexchange.com/questions/25053/best-practical-algorithm-for-sentence-similarity and https://stackoverflow.com/questions/62328/is-there-an-algorithm-that-tells-the-semantic-similarity-of-two-phrases

To determine the similarity of sentences we need to consider what kind of data we have. For example if you had a labelled dataset i.e. similar sentences and disimilar sentences then a straight forward approach could have been to use a supervised algorithm to classify the  sentences. 

An approach that could determine sentence structural similarity would be to average the word vectors generated by word embedding algorithms i.e word2vec. These algorithms create a vector for each word and the cosine similarity among them represents semantic similarity among words. (Daniel L 2017)

Using word vectors we can use the following metrics to determine the similarity of words. 


Cosine distance between word embeddings of the words
Euclidean distance between word embeddings of the words


Cosine similarity is a measure of the similarity between two non-zero vectors of an inner product space that measures the cosine of the angle between them. The cosine angle is the measure of overlap between the sentences in terms of their content. 

The Euclidean distance  between two word vectors provides an effective method for measuring the linguistic or semantic similarity of the corresponding words. (Frank D 2015)

Alternatively you could calculate the eigenvector of the sentences to determine sentence similarity. 

Eigenvectors are a special set of vectors associated with a linear system of equations (i.e. matrix equation). Here a sentence similarity matrix is generated for each cluster and the eigenvector for the matrix is calculated. You can read more on Eigenvector based approach to sentence ranking on this paper https://pdfs.semanticscholar.org/ca73/bbc99be157074d8aad17ca8535e2cd956815.pdf

For source code Siraj Rawal has a Python notebook to create a set of word vectors. The word vectors can then be used to find the similarity between words. The source code is available here https://github.com/llSourcell/word_vectors_game_of_thrones-LIVE

Another option is a tutorial from Oreily that utilizes the gensin Python library to determine the similarity between documents. This tutorial uses NLTK to tokenize then creates a tf-idf (term frequency-inverse document frequency) model from the corpus.  The tf-idf is then used to determine the similarity of the documents. The tutorial is available here https://www.oreilly.com/learning/how-do-i-compare-document-similarity-using-python